In offshore drilling, there is an increasing demand for prolonged service life of the well assembly or equipment and also there is a demand for reduced or eliminated pollution to sea from the drilling activities and reduced equipment weight and cost. In addition, increased versatility of the well assembly or equipment is desirable.
So far there is no good solution for having, in combination, increased service life and versatility as well as no pollution/discharge from the drilling activities.
All drilling operations today from floating MODUs are using a marine riser connecting the MODU with the subsea blow out preventer, BOP. The marine riser has several functions, namely to serve as a return conduit for drilling fluid coming from well, to be used for attachment of rigid choke and kill lines that must be in place between the subsea BOP and the MODU for well control use, to bring the subsea BOP to/from the wellhead on the seabed, and to avoid pollution during drilling. Without a marine riser and where the wellbore is completely open to seawater, there would be pollution of mud/drilling fluids when tubulars are pulled from the drilling mud environment inside the bop and out to the seawater outside. When using of oil based mud (OBM) there will be an interface fraction heavily contaminated with seawater which could be sucked into the mud system when drilling mud is pumped back to the MODU.
Accordingly no drilling is currently attempted from a MODU without the use of a marine riser in order to control the drilling fluid and the hydrostatic head inside the borehole. Further, the modern generation MODU's are built for large water depths and are expensive to operate requiring a high day rate. In order to reduce risk of downtime related to the subsea BOP systems, there are more and more contingencies built into the BOPs itself, such as more BOP closure rams, resulting in taller and much heavier BOP's than before.
Maximum subsea wellhead loading regimes are produced when a subsea drilling BOP stack is installed on top of a production xmastree which in turn is installed on a producing well wellhead. The marine riser is connected to the BOP, causing additional horizontal loading/bending moment to the top of the BOP. In this situation, the wellhead loading regime is at its most severe and imposed bending stress and strains imposed on subsea high pressure wellhead housings are at their highest values.
However, typical subsea wellhead systems were designed for lighter equipment and shorter service life, not foreseeing heavier equipment and extended operation modes. Currently many of the installed wellheads with their xmas trees are heavily overstressed due to prolonged drilling and completion time, making it in many cases risky to connect to them with conventional heavy BOPs and marine drilling riser systems. The risk of total loss of barriers and heavy pollution is then increased. The marine riser with its horizontal and lateral forces increases the stressed loads on wellheads.
The closest prior art documents are as follows: SPE/IAD 130308 Deepwater Riserless Mud Return System for Dual Gradient Tophole Drilling, which merely relates to tophole drilling, US 2008/190663 A1, US 2008/190663 A1, U.S. Pat. No. 6,230,824, all of which are only of minor relevance, including no teaching helping the person of ordinary skill in the art to solve the well control, overstress/fatigue and pollution problems associated with drilling a complete subsea well from a floating MODU.
Other riserless drilling concepts have been proposed, such as described in publications U.S. Pat. Nos. 6,648,081 and 6,415,877, however introducing a subsea rotating control device (RCD) or a rotating BOP (RBOP) on top of the subsea BOP, which permanently is closed around the drillpipe creating a pressure tight barrier between the seawater (pressure) and the wellbore below. The outlet from the wellbore to the pump system is here below the BOP on the wellbore annulus. Such device (RCD) or R BOP have a finite life span and are subject to frequent failure due to wear during drilling and tripping. Having to change these elements in deep waters has huge associated costs and well control risks. Also it is unknown what to do with the contaminated mud/seawater fluids during such operations. Tripping out of the well with casings, drillpipe, completion strings, etc, is therefore not recommended or possible.